rsync-backup.in, rsync-backup.8: Trim caches on fshash mismatch.

rsync-backup.in, rsync-backup.8: Trim caches on fshash mismatch.

  * Capture the diff output in a file so we can pass it onto fshash for
    cache pruning if it's not empty.

  * If we're retrying because of an fshash mismatch then clear cache
    entries.  This is delayed from the cache-mismatch processing because
    we need the snapshot mounted so that fshash can do name->inode
    mapping.

  * By default, retry once.  Otherwise we don't get any benefit from
    evicting maybe-stale entries.